[libc] Properly call init functions and main through GOT

In the CRT startup code, when linked in a PIC executable, jumps to
`__init_libj6`, `__init_libc`, `main`, and `exit` were not linked
correctly. They needed a bit more support for looking up the GOT, and
getting the symbol address out of it.

Now libutil has a `got.inc` file for inclusion in asm code that needs to
reference symbols from the GOT.

src/libraries/libc/arch/amd64/crt/crt0.s

@@ -1,3 +1,5 @@
+%include "util/got.inc"
+
 extern main
 extern exit
 extern __init_libj6
@@ -12,17 +14,21 @@ _libc_crt0_start:
     push 0
     mov rbp, rsp
 
-    call __init_libj6 wrt ..got
+    lookup_GOT __init_libj6 
+    call rax
     mov rbx, rax
 
-    call __init_libc wrt ..got
+    lookup_GOT __init_libc
+    call rax
 
     mov rdi, 0
     mov rsi, rsp
     mov rdx, 0    ; TODO: actually parse stack for argc, argv, envp
     mov rcx, rbx
-    call main wrt ..got
+    lookup_GOT main
+    call rax
 
     mov rdi, rax
-    call exit wrt ..got
+    lookup_GOT exit
+    call rax
 .end:

src/libraries/util/include/util/got.inc

@@ -0,0 +1,7 @@
+extern _GLOBAL_OFFSET_TABLE_
+
+; Put the address of the given symbol in rax
+%macro lookup_GOT 1
+    lea rax, [rel _GLOBAL_OFFSET_TABLE_]
+    mov rax, [rax + %1 wrt ..got]
+%endmacro